THE PRESENCE OF 2 TYPES OF BETA-CYANOALANINE SYNTHASE IN GERMINATING-SEEDS AND THEIR RESPONSES TO ETHYLENE

Germinating seeds of many species contain two types of beta-cyanoalanine synthase (GAS, EC 4.4.1.9) that convert HCN to beta-cyanoalanine. One is cytoplasmic CAS (cyt-CAS), which is precipitated by 50 to 60% (NH4)(2)SO4 and has a pH optimum of 10.5. Cytoplasmic CAS is present at high levels in dry seed and its activity does not increase during imbibition. The activity of cyt-CAS is not affected by exogenously applied ethylene (C2H4), except in rice (Oryza sativa cv. Sasanishiki). The second type of CAS found in seed is mitochondrial CAS (mit-GAS), which is precipitated by 60 to 70% (NH4)(2)SO4 and has a pH optimum of 9.5. Mitochondrial GAS is present at low levels in dry seed, and its activity increases greatly during imbibition in the seeds of all species tested. Exposure to C2H4 stimulated mit-GAS activity in seeds of rice, barley (Hordeum vulgare cv. Hadakamugi), cucumber (Cucumis sativus cv. Kagafushinari) and cocklebur (Xanthium pennsylvanicum). The increase in the mit-GAS activity in cocklebur in response to C2H4 commenced after a lag period of 2 to 3 h when the duration of soaking was short (16 h), but commenced without a lag period when the seeds were soaked for three months. Application of both chloramphenicol and cycloheximide to the axial and cotyledonary tissues of cocklebur seeds strongly inhibited growth as well as the increase in mit-GAS activity. It is postulated that the mit-GAS is synthesized de novo during imbibition and that its activity is regulated by C2H4. CO2, which also promotes seed germination in some species, was ineffective in stimulating mit-GAS activity in cocklebur seeds.